Each year nicotine addiction is responsible for more than 125,000 deaths of American women. In 1987, cigarette smoking related lung cancer surpassed breast cancer as the leading cause of death by cancer among women. The prevalence of smoking has declined more slowly for women than for men, suggesting that quitting smoking is more difficult for women. Factors that may contribute to the gender difference in cessation rates include women's greater tendency to smoke as a means of coping with negative affect, and their greater concern about postcessation weight gain. It seems clear that special interventions are needed to address these unique concerns of female smokers. Nicotine replacement therapy has shown some success in improving smoking cessation rates, reducing the severity of negative affect usually experienced during cessation, and, in the case of nicotine gum, minimizing postcessation weight gain. Aerobic exercise has also been found to improve mood and control weight. In combination, nicotine replacement therapy and aerobic exercise should be a powerful smoking cessation treatment for women. The proposed study will investigate the effects of an aerobic exercise intervention as an adjunct to nicotine polacrilex gum therapy. Three hundred female smokers will receive nicotine gum therapy and will be randomly assigned to an exercise intervention, an equal contact control condition, or a gum alone control condition. The exercise intervention will consist of three 45-minute sessions of aerobic exercise per week from 3 weeks precessation through 16 weeks postcessation. All participants will be followed for one year after cessation. In addition to determining the effectiveness of the adjunct exercise intervention on cessation rates, the mechanisms (e.g., relief of negative moods, suppression of cessation-related weight concerns, relief of premenstrual distress) by which exercise affects cessation will be examined. The proposed study will provide the first large-scale test of a very promising intervention to aid women in smoking cessation. The combination of an exercise intervention with nicotine replacement, which has not yet been investigated, should provide a particularly effective treatment program for female smokers. Intensive focus on the mechanisms by which exercise affects cessation will provide information essential both for understanding the nature of the relationship between exercise and smoking cessation, and for later refinement and enhancement of the exercise intervention.